Is it possible to control dynamic bevel segments with a stylus?
Found it. For Blender 2.79 the Numpad Minus and Numpad Plus also do what the missing scroll wheel does.

For Blender 2.8, When you are in the Bevel tool, press S (for Segments). At the bottom of the screen there is a list of all Bevel Properties that can be triggered with hotkeys. You can switch to different properties at will and the last setting will be set for you.